This Part Fits the following kits / parts:

#108728 - HEAVY-DUTY PINION GEAR 15T (8mm BORE/1.5M)

Baja 5B FLUX

Fine-tune the speed and acceleration of the Baja 5B Flux with a
selection of Heavy-Duty pinion gears. Use a larger pinion gear for
higher top speeds, or use a smaller pinion gear for increased
acceleration.